A professional driver who accels at high performance maneuvering, chase, pursuit, evasion, and high speed driving.
A driver that understands the mechanics of driving as an art form of finesse between the wheel, shifting gears, brake, and throttle control.
A person who can perform tricks neccesary to keep a vehicle moving such as J-turns, continuous donuts, burnouts (with a manual tranmission), double-clutching, 180's, 360's, drifting, etc.
A pro who understands and utilizes over and under steer, pitch, roll, brake-steering, throttle steering etc.
A person who has the knowledge to drive any vehicle at any time without hesitation, whether it be a passenger vehicle, front wheel, rear wheel, all-wheel drive (using them to their specific advantages) trucks, motorcycles, commercial vehicles, industrial, and agricultural equipment.
A driver who can manage a vehicle that is probelmatic or otherwise difficult to drive. i.e. shifting or stalling problems, flat tire(s) etc.

Derived originally from car thiefs which necessitated being able to drive any vehicle (without prior knowledge of it) and driven in a hurry.

See movies: The Transporter, Gone in 60 Seconds.

John: "Its a 15 minute run from dragonfire crossing to lookout mountain road, but we need it down to 12 minutes. Police are lining the road, the clutch is hot and slipping, and Rich can't drive like that with a cast on his arm. Who are we going to get?
Ken: "Lets call KRASH! ... he's a wheelman."

Corrections from previous definition:
Also known as Wheelfuck or douche to complete and utter idiots (See: "wankers, tossers"); A person who likes to win a slander war, spam, and throw fruit at the loser of the slander war.

I would eat that apple but I have the suspicion that Wheelman owned someone with it.